1. Field of the Invention
The invention relates to a method of controlling fluid loss from a thixotropic cement slurry of the type used in cementing conduit in a wellbore.
2. Description of the Prior Art
Thixotropic cement slurries have been used in cementing wells for a number of years, particularly on wells where lost circulation is likely to be a problem. Among those thixotropic cement systems which have enjoyed the most commercial success is that disclosed by Spangle in U.S. Pat. No. 3,563,313, the teachings of which are expressly incorporated herein.
A chronic problem with such slurries, however, is that such slurries have a high degree of fluid loss. This problem is recognized, for example, in SPE Paper 6011. At column 2, line 3 of Spangle, it is suggested that fluid loss control compositions may be used in the thixotropic cement composition there described. However, although a slight measure of fluid loss control can be imparted to the slurry itself using some convention additives, the potential problems are such that such an approach is not commercially practical. For example, cellulose gums incorporated in the slurry provide a slight measure of fluid loss control, but are also somewhat detrimental to the thixotropic properties of the cement, to the extent that they do not provide a commercially practical solution to the problem. Thus, a slurry comprised of Class A portland cement, and, by weight of cement, 12% CaSO.sub.4.1/2H.sub.2 O, 0.4% hydroxyethylcellulose, 60% water, and 3% calcium chloride flake (assay 77-80% CaCl.sub.2) had an API fluid loss of 230 ml/min at 80.degree. F. and 1000 psi. Larger amounts of hydroxyethylcellulose substantially destroyed the thixotropic properties of the slurry. Polyamines and polyalkylenepolyamines are not themselves harmful to the thixotropic properties of the cement, but are effective fluid loss control agents only when used in combination with a dispersant (U.S. Pat. No. 3,491,049), and the dispersant destroys the thixotropic properties of the slurry. Accordingly, the need for a suitable means for controlling fluid loss from such cement slurries has persisted.
In commonly assigned application Ser. No. 766,835 filed Feb. 7, 1977 by Thomas J. Griffin, Jr., and Larry K. Moran, now U.S. Pat. No. 4,083,407 issued Apr. 11, 1978, there is disclosed a spacer composition which, in a preferred embodiment, is that employed in the present invention. It was the present applicants, however, who first recognized that said spacer could be used to control fluid loss from a thixotropic cement slurry.